[mlir][sparse] auto-insertion of conversion to resolve cycles

[mlir][sparse] auto-insertion of conversion to resolve cycles

When the iteration graph is cyclic (even after several attempts using less and less constraints), the current sparse compiler bails out, and no rewriting hapens. However, this revision adds some new logic where the sparse compiler tries to find a single input sparse tensor that breaks the cycle, and then adds a proper sparse conversion operation. This way, more incoming kernels can be handled!

Note, the resulting code is not optimal (although it keeps more or less proper "sparse" complexity), and more improvements should be added (especially when the kernel directly yields without computation, such as the transpose example). However, handling is better than not handling ;-)

Reviewed By: bixia

Differential Revision: https://reviews.llvm.org/D128847